Lock mandrels are designed to engage corresponding landing nipples, located at predetermined positions within the tubing string and designated by size of seal bore.
A typical well completion would consist of several landing nipples made up to the tubing at various depths. The nipples would decrease in seal bore diameter the deeper they are positioned in the tubing string. A landing nipple typically consists of an annular recess to accept radially expandable locking dogs forming part of the lock mandrel, a no-go shoulder against which a corresponding no-go shoulder of the lock mandrel would normally seat in use, and a seal bore.
A typical lock mandrel running procedure would involve the lock mandrel being screwed on to the top of the flow control device (e.g. plug, valve, etc) which is to be installed. A special running tool would be attached to the lock mandrel with shear pins. The assembly is then run into the tubing at the end of a wireline toolstring. When the no-go shoulder of the mandrel hits the corresponding no-go shoulder of the landing nipple, the lock mandrel will stop in proper alignment within the landing nipple. At this point the locking dogs of the mandrel are in their retracted position and are aligned with the locking recess in the landing nipple, and the V-packing stack of the mandrel, if employed is located in the seal bore.
In order to lock and leave the lock mandrel in place, a wireline toolstring is used to jar down in order to shear a set of shear pins in the lock mandrel so as to move the locking dogs into their extended position such that they engage the annular recess of the landing nipple. The running tool is then disengaged from the lock mandrel by jarring upwards to shear a second set of shear pins.
When a lock mandrel is set into a landing nipple in this manner a potential problem arises if pressure is applied from above. The lock mandrel effectively rests on the no-go shoulder of the landing nipple, which has a relatively small area in order to present as little restriction as possible to fluid flow through the nipple. When pressure is applied at the surface against the lock with a plug attached, the load on the nipple no-go shoulder is very high, and excessive pressure can cause damage. Accordingly, all landing nipples are pressure-rated and the rated pressure must not be exceeded.
Previous attempts to up-rate landing nipples having a given no-go shoulder area have centred upon arranging for the load to be carried on the relatively larger contact area between the mandrel locking dogs and the corresponding nipple recess. Such approaches have involved arrangements with deformable no-go shoulders, arrangements in which expansion of the locking dogs lifts the mandrel no-go shoulder off the nipple no-go shoulder, or replacing the no-go shoulder with collapsible fingers. Such approaches have various operational difficulties in practice.